Turbine engines, and particularly gas or combustion turbine engines, are rotary engines that extract energy from a flow of combusted gases passing through the engine onto a multitude of rotating turbine blades.
Engine efficiency increases with temperature of combustion gases. However, the combustion gases heat the various components along their flow path, which in turn requires cooling thereof to achieve a long engine lifetime. Typically, the hot gas path components are cooled by bleeding air from the compressor. This cooling process reduces engine efficiency, as the bled air is not used in the combustion process.
Turbine engine cooling art is mature and includes numerous patents for various aspects of cooling circuits and features in the various hot gas path components. For example, the combustor includes radially outer and inner liners, which require cooling during operation. Turbine nozzles include hollow vanes supported between outer and inner bands, which also require cooling. Turbine rotor blades are hollow and typically include cooling circuits therein, with the blades being surrounded by turbine shrouds, which also require cooling. The hot combustion gases are discharged through an exhaust which may also be lined, and suitably cooled.
In all of these exemplary turbine engine components, thin metal walls of high strength superalloy metals are typically used for enhanced durability while minimizing the need for cooling thereof. Various cooling circuits and features are tailored for these individual components in their corresponding environments in the engine. In addition, all of these components typically include common rows of film cooling holes.
A typical film cooling hole is a cylindrical bore inclined at a shallow angle through the heated wall for discharging a film of cooling air along the external surface of the wall to provide thermal insulation against the flow from hot combustion gases during operation. The film is discharged at a shallow angle over the wall outer surface to minimize the likelihood of undesirable blow-off, which would lead to flow separation and a loss of the film cooling effectiveness.
A coating, such as a thermal barrier coating can be placed on portions of the film cooling hole to prevent thermal damage due to high temperatures.